Heated Windshield Z2, function

WARNING: This page is about a different car, the 2008 Audi S4, 2008 Audi RS 4, and 2008 Audi A4. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Identifying Heated Windshield Control Module J505 & Climatronic Control Module J255

Courtesy of VOLKSWAGEN UNITED STATES, INC.

The Heated Windshield Control Module J505 - B - is activated by the operating and display module, Climatronic Control Module J255 (reading measuring value block, display group "015") → Read Measuring Value Block (function 08) :

After starting cold engine if ambient temperature is less than + 5 °C and operating and display module, Climatronic Control Module J255 is set to "Auto" or "Defrost" mode. The temperature setting on the operating and display module, Climatronic Control Module J255 must be such that the air conditioner is in heating mode and the temperature measured by the sensors (Left Vent Temperature Sensor G150, Right Vent Temperature Sensor G151, Left Footwell Vent Temperature Sensor G261 and Right Footwell Vent Temperature Sensor G262) must not be in excess of +35 °C. The heating period is governed by ambient temperature (max. 6 minutes at -40 °).
On selecting "Defrost" mode on the operating and display module, Climatronic Control Module J255 if the ambient temperature is less than + 5 °C. The heating period is between 2 and 6 minutes depending on ambient temperature. As long as the Heated Windshield is activated, the indicator lamp in the Defrost button flashes (only if, on pressing the Def button, the Heated Windshield had not already been activated by the operating and display module, Climatronic Control Module J255). Activation is only effected if the engine is running and the activation conditions described above are fulfilled.

The Heated Windshield Control Module J505 - B - switches on the Heated Windshield if the following conditions are fulfilled:

Operating and display unit, Climatronic Control Module J255 activating Heated Windshield Control Module J505
Voltage at Heated Windshield Control Module J505 greater than 12.70 V
Heated Windshield Control Module J505 not overheated

The Heated Windshield Control Module J505 switches approx. Battery voltage to the operating and display module, Climatronic Control Module J255. This informs the operating and display module, Climatronic Control Module J255 that the Heated Windshield Control Module J505 is ready to switch on the Heated Windshield. If this voltage is halved by the operating and display module, Climatronic Control Module J255 (to approx. 5 to 8 V), the Heated Windshield Control Module J505 switches on the heating.
The resistance of the metal foil in the Heated Windshield Z2 is approx. 1.9 Ω. A voltage greater than the electrical system voltage is required to attain the maximum possible heat output of approx. 1000 W (watts) at the windshield. The Heated Windshield Control Module J505 converts the electrical system voltage into a variable voltage of between 25 and 48 V depending on electrical system voltage. If the voltage at the Heated Windshield Control Module J505 is greater than 13.10 V, the Heated Windshield Z2 is activated with a voltage greater than 41 V (up to max. 48 V). In the event of problems relating to poor Heated Windshield Z2 heat output, check the voltage at the two connection points of the two wires at the windshield. To do so, actuate the Heated Windshield Z2 for example via the "Output Diagnostic Test Mode (DTM) diagnosis" function with the engine running and the Battery fully charged (electrical system voltage must be greater than 13.1 V).
To prevent electrical system overload, the Heated Windshield Control Module J505 regulates the heat output of the Heated Windshield. With an electrical system voltage of 12.70 V this is approx. 200 W and is infinitely regulated to 1000 W at 13.10 V. The Heated Windshield Control Module J505 permits an increase in power output of max. 200 W per second to guard against voltage dips in the electrical system on switching on the Heated Windshield.
To prevent activation of the Heated Windshield Z2 in the event of a short circuit or an open circuit in the wiring, the Heated Windshield Control Module J505 checks the resistance of the Heated Windshield Z2. If the measured resistance is less than approx. 1.5 Ω (short circuit) or greater than approx. 3.3 Ω (open circuit or contact resistance), the Heated Windshield Control Module J505 does not switch on the heating or switches it off. Check the wiring between the Heated Windshield Control Module J505 and the Heated Windshield Z2 for contact resistance or short circuit. The resistance measured between the two wires from the Heated Windshield Control Module J505 (to the Heated Windshield Z2) must be less than 2.5 Ω and greater than 1.4 Ω